Hydrocarbon fractions produced in the petroleum industry are typically contaminated with various sulfur impurities. These hydrocarbon fractions include diesel fuel and gasoline, including natural, straight run and cracked gasolines. Other sulfur-containing hydrocarbon fractions include the normally gaseous petroleum fraction as well as naphtha, kerosene, jet fuel, fuel oil, and the like. The presence of sulfur compounds is undesirable since they result in a serious pollution problem. Combustion of hydrocarbons containing these impurities results in the release of sulfur oxides which are noxious and corrosive.
Federal legislation, specifically the Clean Air Act of 1964 as well as the amendments of 1990 and 1999 have imposed increasingly more stringent requirements to reduce the amount of sulfur released to the atmosphere. The United States Environmental Protection Agency has lowered the sulfur standard for diesel fuel to 15 parts per million by weight (ppmw), effective in mid-2006, from the present standard of 500 ppmw. For reformulated gasoline, the current standard of 300 ppmw has been lowered to 30 ppmw, effective Jan. 1, 2004.
Because of these regulatory actions, the need for more effective desulfurization methods is always present. Processes for the desulfurization of hydrocarbon fractions containing organosulfur impurities are well known in the art. The most common method of desulfurization of fuels is hydrodesulfurization, in which the fuel is reacted with hydrogen gas at elevated temperature and high pressure in the presence of a costly catalyst. U.S. Pat. No. 5,985,136, for example, describes a hydrodesulfurization process to reduce sulfur level in naptha feedstreams. Organic sulfur is reduced by this reaction to gaseous H2S, which is then oxidized to elemental sulfur by the Claus process. Unfortunately, unreacted H2S from the process is harmful, even in very small amounts. Although hydrodesulfurization readily converts mercaptans, thioethers, and disulfides, other organsulfur compounds such as substituted and unsubstituted thiophene, benzothiophene, and dibenzothiophene are difficult to remove and require harsher reaction conditions.
Because of the problems associated with hydrodesulfurization, research continues on other sulfur removal processes. For instance, U.S. Pat. No. 6,402,939 describes the ultrasonic oxidation of sulfur impurities in fossil fuels using hydroperoxides, especially hydrogen peroxide. These oxidized sulfur impurities may be more readily separated from the fossil fuels than non-oxidized impurities. Another method involves the desulfurization of hydrocarbon materials where the fraction is first treated by oxidizing the sulfur-containing hydrocarbon with an oxidant in the presence of a catalyst. U.S. Pat. No. 3,816,301, for example, discloses a process for reducing the sulfur content of sulfur containing hydrocarbons by oxidizing at least of portion of the sulfur impurities with an organic hydroperoxide such as t-butyl hydroperoxide in the presence of certain catalysts. The catalyst described is preferably a molybdenum-containing catalyst.
We have found that although titanium-containing catalysts are effective at oxidizing sulfur impurities in hydrocarbon fractions, the catalyst is prone to deactivation due to the presence of nitrogen-containing impurities in the hydrocarbon fraction.
In sum, new methods to oxidize the sulfur compound impurities in hydrocarbon fractions are required. Particularly required are processes which effectively oxidize the difficult to oxidize thiophene impurities. We have discovered that the process for oxidizing organosulfur impurites found in fuel streams is improved by first removing organonitrogen impurities from the fuel stream.